NEBULA. 31 



—an apparent distribution whicli must not, liowever, be con- 

 founded with their actual distribution through the region? of 

 Epace. We now, therefore, proceed to the consideration of 

 the remarkable differences presented by their individual forms, 

 which are either regular (globular, more or less elliptical, an- 

 nular, planetary, or resembling the photosphere surrounding 

 a star) or irregular, and almost as difficult to classify as those 

 of the aggregated aqueous vapor of our atmosphere — the 

 clouds. The elliptical (spheroidal) form^ has been regarded 

 as the normal type of nebulae ; this form is most readily re- 

 solved into clusters of stars Avhen it assumes a globular shape 

 in the telescope ; but when, on the other hand, with instru- 

 ments of equal powers, it appears much flattened, elongated 

 in one dimension, and discoidal, it is less easy of resolution.! 

 G-radual transitions of form from the round to the elongated, 

 elliptical, or awl-shaped form, are of frequent occurrence in 

 the heavens. {Fhilos. Transact., 1833, p. 494, pi. ix., figs. 

 19—24.) The nebula is always condensed around one or more 

 central points (nuclei). It is only by a discrimination between 

 round and oval nebulce that we recognize double nebulce ; for 

 as no relative motion is perceptible among the individual neb • 

 ulous bodies, either in consequence of its absence or its ex- 

 treme slowiiess, we are deficient in a criterion by which ta 



p. 79.) This early recognition of binaiy systems, long before that of ^ 

 Ursae Maj. {Cesmos, vol. iii., p. 185), is the more remarkable, as Lacaille, 

 seventy years later, did not describe a Crncis as a double star; perhaps 

 (as Riimker conjectures), because the main star and the companion were 

 then not sufficiently distant from each other. (Compare Sir John Her- 

 Bchel, Observacions at the Cape, § 183-185.) Eichaud also discovered 

 the binary character of a Centauri almost simultaneously with that of a 

 Crucis, and fully nineteen years before the voyage of Feuillee, to whom 

 Henderson erroneously attributed the discovery. Richaud remarks 

 "that, at the time of the comet of 1689, the two stars which form the 

 double star a Ci'ucis were at a considerable distance from eacli oti'er; 

 but that in a twelve-feet refractor both parts of a Centauri could be dis- 

 tinctly recognized, and appeared to be nearly in contact. 



* Observations at the Cape, § 44, 104. 



t Cosmos, vol. iii., p. 140, and note. As we have already remarked iia 

 I'eference to clusters of stars {Ibid., p. 143), Mr. Bond, of the United 

 States, succeeded, by means of the great space-penetrating power of 

 his refractor, in completely resolving the very elongated, elliptical neb- 

 ula of Andromeda, which, according to Bouillaud, had been already 

 described before the time of Simon Marius in 985 and 1428. It has a 

 reddish light. Near this celebrated nebula lies the still unresolved, 

 but very eimilarly shaped nebula, discover! >d on the 27tli of August, 

 1783, bj my honored IViend, Miss Caroline Herschel, who died at au 

 adva')*"-^ age, universally esteemed. {Philos. Transact., 1833, No. 6J 

 of th<» tlatvalogue of Nebulce, fig. 52.) 



